The invention relates generally to the formation of molded parts. More specifically, the invention provides improved methods and apparatus for the formation of parts from plastics and similar mold materials by injection molding.
Injection molding is a commercially important technique used in the fabrication of parts from plastic and other mold materials. According to well known techniques, molten plastic or another mold material is forced by a ram through one or more gates into a mold cavity formed in the shape of the desired part. The ram continues forcing mold material through the gate until the mold cavity is filled and the mold material is packed under pressure inside the mold cavity. As the mold material cools, it solidifies in the shape of the mold cavity. When the material is sufficiently cool, the formed part is removed from the mold cavity and the apparatus is made ready for another molding cycle.
Multi-gated mold cavities are often used in the production of parts having complex shapes. As the name suggests, a multi-gated mold cavity includes multiple gates through which the mold material is forced into the mold cavity. Mold material is most commonly forced by a single ram through a common manifold serving all of the gates and through each of the gates into the cavity. As with a single-gated mold cavity, mold material is forced into the mold cavity until the mold material is filled and packed inside the mold cavity. A "knit line" is formed at the interface where mold material flowing from a given gate meets and joins with mold material flowing from a different gate.
The properties and positions of knit lines are important considerations in the performance and appearance of injection molded parts. A knit line formed during injection molding according to known techniques is typically much less strong than the mold material itself. For this reason, it is important that a knit line not be located in a region of high stress in the finished part. Additionally, it may be desirable to control the location of the knit line in order to enhance the appearance of the finished part.
The location of a knit line in a finished part can be controlled to some degree by changing the relative speed at which mold material flows through the individual gates of a multi-gated cavity. Previously, this was most commonly done by machining the gates to change the size of the gate openings. However, this approach is expensive and time consuming and does not allow for quick adjustments to be made between mold cycles, and certainly not during a single mold cycle.
A somewhat more flexible approach to flow control through individual gates of multi-gated cavities is disclosed in U.S. Pat. No. 5,141,696 to Osuna-Diaz. The Osuna-Diaz patent discloses a molding machine in which a separate manually adjustable valve is disposed within the flow channel that feeds each gate. By adjusting the valve, a user of the machine can adjust the rate at which mold material flows through the corresponding gate. According to the patent, this allows the user to "balance" the flow through the gates by adjusting the valves between mold cycles. Although the patent does not discuss it specifically, the apparatus could be used to control the location of the knit line by adjusting the rates of flow through the gates relative to one another. However, the Osuna-Diaz patent does not disclose any apparatus suitable for dynamically adjusting the flow of material through the gates during a single mold cycle.
In addition to issues related to knit line location, problems sometimes occur that arise from shrinkage and warping of parts during the filling, packing and cooling stages. To achieve greater reliability and repeatability between molding cycles, researchers have sought means for controlling certain conditions within the mold cavity during the molding cycle. An investigation of this type is reported in Chiu, Wei, and Shih, "Adaptive Model Following Control of the Mold Filling Process in an Injection Molding Machine," Polymer Engineering and Science, vol. 31, no. 15, pp. 1123-29 (mid-August 1991).
In the Chiu investigation, a molding machine was fitted with a pressure transducer for reading pressure within the mold cavity during the molding operation. A servo-hydraulic system controlled by a computer was also added for controlling the pressure exerted by the injection ram. The computer was programmed to control the ram pressure based on signals from the pressure transducer. In this way, the pressure at one point inside the cavity during the molding process could be indirectly controlled in an attempt to follow a predetermined optimal pressure profile.
More or less continuous control of a machine parameter, e.g. ram pressure, during a mold cycle based on a reading of a condition from inside the mold cavity, e.g. cavity pressure, can be referred to as "closed-loop" control. Closed-loop control can significantly reduce variations between individual mold cycles. Additionally, closed-loop control provides for a greater degree of flexibility because the machine parameters can be adjusted during an individual mold cycle.
The apparatus disclosed in Chiu is less than ideal however, in that controlling the ram pressure controls the cavity pressure only indirectly. It would be advantageous to provide more direct means for controlling the flow of mold material through the gates so that conditions inside the mold cavity would be more directly controllable. It would be more advantageous if such means provided for effective closed loop control of the flow of material through the gate. Finally, it would be very advantageous if such means for controlling the flow of material through the gate could be adapted to provide for individual control of material flow through each of the gates of a multi-gated mold cavity. The invention provides methods and apparatus having these and other advantages, as will be described more fully below.